Recovery of chemicals in wood pulp preparation



K. R. GRAY EIAL 2,772,965

RECOVERY or CHEMICALS IN wooo PULP iREFARATION 2 Sheets-Sheet ,1

Dec. 4, 1956 Filed June 2, 1953 METER READING 0 Row Block Liquor Absorbent Containing Vololizod Sulfide 6 Carbonated Liquor EQUIVALENT ml; 1.0 N. Aq N03 INVENTORS.

Kennefrh Russell Gray Horizell Lance Crosby BY John Charles Sreinberg a 53 M0701. arr/M172?" ATTORNEYS United States Patent RECOVERY OF CHEMICALS IN WOOD PULP PREPARATIQN Kenneth Russell Gray, Hartzell Lance Crosby, andJohn Charles Steiuberg, Shelton, Wasln, assignors'to Rayonier Incorporated, Shelton, Wash, a corporation of Delaware Application lune 2, 1953, Serial No. 359,146

19 Claims. (Cl. 92-2) ment of the waste liquor to lower the sulfur content prior to evaporation and burning, and for the separation from the liquor of an alkali-type lignin.

In theconventional operations for the recovery' of chemicals from alkaline waste liquor containing a subs'tantial amount of residual sodium sulfide, such as a kraft black liquor, hydrogen sulfide is released in dilute;

form during evaporation resulting in an uneconomical loss of sulfur, and creating undesirable atmospheric pollution. Such loss of sulfide sulfur in the form of very dilute hydrogen sulfide occurs particularly in the final stages of evaporation which is usualy effected by direct contact of the liquor with flue gas. Further, kraft' pulping operations have been limited to the use of digestion liquors having a relatively low sulfide content in order to produce a smelt with a sufiiciently low proportion of sodium sulfide to produce a smelt with a low melting;

point.

Recent proposals have been made to reduce sulfur lossin the evaporation ofkraft black liquor by oxidation of potentially volatile sulfide sulfur to nonvolatile forms such as-thiosulfate and sulfate; Such proposals, however, do not provide the low sulfur-to-sodium ratio, which must bemaintained in order to get a smelt with a practically low melting point.

- I'n norm'al kraft pulping operations, the ratio of sulfur to'sodium is sufiicient'ly low that smelts of a. practical melting point(l500-1700 F.) are obtained. However, in producing special grades of cellulose,- such asdissolving pulps, itmay be' desirableto operate with a sulfur-tosodium ratioconsiderably higher than that normally used;

The smelt obtained from the furnacing of black liquor resulting from this process will contain a very high proportion of sodium sulfide and consequently .will have- I an undesirably high melting point.

In accordance with the improved process of 'ourinvention, we subject the sulfide-containing waste liquors, such asthe black liquors from kraft pulping operations, and the like, to a pressure carbonation treatment followed by stripping under a vacuum to remove avery appreciable proportion of the sulfide sulfur as hydrogen sulfide, and simultaneously to separate from the liquor a lignin containing but a small amount of sodium and a small amount of organically combined sulfur. applicable to the treatment of black liquors from kraft pulping of the conventional type, our process may be advantageously applied to alkaline waste liquorsysuch as result when the alkaline digestion has followed a prehydrolysis treatment of' thewood. The carbonation process may also be applied-to sulfide-containingblack liquors'from the alkaline refining of wood pulps which have already been substantially delignified in a prior treatment, such as-by acid sulfite digestion. In thiscase, little or no lignin will precipitate out but nevertheless the advantages of the invention will still be obtained with regard to recovering concentrated hydrogen sulfide, minimizing sulfur lossesand improving the flow properties of the resulting smelt in cases where these liquors are furnaced.

Wood digestion with liquors containing considerable sulfur will result in a higher than normal sulfur content= in the black liquor. Increasing the sulfur-to-sodaratio of the liquor will proportionally increase the sulfide content ofthe resulting smelt. The presence. ofthis sodium: sulfidewith a melting'point ofabout122.00," F. necessitates high furnace operating temperatures and a.corre-. spending, increase in furnace maintenance problems. Fume lossfrom the furnace isalso greatly increased.

That a-very high content ofsodiunr sulfide will result in aicomposition of extremely'high melting pointtwillbe evident from. Table'I which illustrates the variationszin: fusion temperatures of various mixtures of NazSv and NazCOa. In column 3, the composition of themixtures of column 1 has been calculated to express the total NazO present as; NazS, which for this two component: system is;equivalent to-the sulfidity;

TABLE I Percent Fusion of Total Percent N e25 in Na S,NagCO3.Mixture.- Point, O. NazO- Present as Na'zS In-actualsmelting operations, the smelt will also contain'appreciable amounts of sodiumsulfate and minor amounts of other sodium salts. These will tendto lower somewhat the fusion points given above for mixturesof pure sodium carbonate and sodium sulfide. For example, in one sample of smelt in which the percentage of the total NazO present as NazS was 45.0%; the- NazCOs, 45.0% and NazSO-r, 10.0%, the fusionpoint was 73 8 C. Inanothersample of smelt in which the percentage of the total NazO present as NazS was-70.0%; the NazCOs, 20%; and Na2SO4, 10.0%, the fusion point-was 777j C.-

As' used in kraft pulping practice, and as used herein, theterm sulfidity? refers to the ratioof NazS' to the sum of. NazS+NazCOa all expressed on an NazO (i. e.

- equivalent) basis.

I clear, it is believed to be due to the fact that in addi- In addition to being tion'to the preponderance of sodium carbonate and sodium sulfide there will always be present smaller amounts of other sodium chemicalsv such as sodiumsulfate, thiosul fate, and, if salt-water floated wood has been, used, sodium chloride. These will modify the properties, of the smelt. Y

Patented} De 4; 1956 Our process provides an improved process comprising the treatment of kraft type waste black liquor with carbon dioxide under pressure (carbonation) and at a sufiiciently low temperature that the lignin which separates out will remain dispersed in semi-colloidal form. This is followed by stripping under a vacuum to release hydrogen sulfide in concentrated form, the solution still being below the sintering temperature of the lignin, preferably followed by increasing the temperature in order to partially agglom smelt and the hydrogen sulfide for conversion to cooking liquors.

When the carbonation is carried out at a temperature below the apparent sintering point of the lignin in aqueous solution, the precipitate will be in a semi-collodial form, and it will generally be convenient to keep it in this form until after the stripping operation. In order to obtain a precipitate which can be readily filtered and washed, the solution is then heated to or slightly above the sintering point for a brief period to partially agglomerate the lignin particles, following which the solution is cooled to a temperature below the sintering point whereupon the lignin may be readily separated from the liquor by filtration.

Alternatively, the carbonation may be carried out at a temperature above the apparent melting point of the lignin. In such case, the lignin which separates out will melt to a heavy oily liquid or tar which can be removed as a liquid dreg from a separating chamber. It is frequently advantageous to efiect such separation of fluid lignin prior to the stripping operation.

As used above apparent sintering or apparent melting points are the sintering and melting points of the precipitated lignin in the presence of liquor, said sintering or melting points being of the order of 70 C. to 90 C. depending upon the type of wood used, rather than the values of 140 C. to 210 C. obtained for completely sodium-free alkali-type lignins in the dry form.

. The process of the invention gives several important advantages including removal from the liquor of potentially volatile sulfide sulfur with recovery in the form of concentrated hydrogen sulfide which may be conveniently reused in the pulping process; reduction of the sulfidity of the black liquor, improving furnace operation and reducing stack losses due to lowering of the melting point of the smelt; permitting, where desired, carrying out kraft pulping at higher sulfidities than have heretofore been practical from the standpoint of chemical recovery; minimizing atmospheric pollution resulting from H28 and mercaptans in stack gases; permitting simultaneous economical recovery of a relatively pure, alkali-type lignin as a salable product, and efiicient means of burning the separated lignin if desired. r

The process of the invention will first be described with. reference to the treatment of a typical kraft digestion waste black liquor. While our process is applicable to the treatment of liquor containing the usual amount of sodium sulfide, it is advantageously applicable to the treatment of liquors containing considerably more sodium sulfide than could be used in present operations. This is due to the fact that we can effectively reduce the sulfur content substantially, the exact value being dependent on the content of residual sulfide, thereby giving a lower melting point smelt. (In some conventional kraft cooks the reduction in sulfur content of the efiluent by the process of the invention was of the order of 50-60%.)

Subject to some variations, as will appear from the following discussion, we prefer to subject the black liquor to a single stage of pressure carbonation and vacuum stripping, as described in our abandoned application Serial No. 166,683, filed June 7, 1950, which subject matter is now incorporated in application Serial No. 415,754, filed March 12, 1954.

In a preferred method of operation, the carbonation treatment is generally effected with any suitable form of carbon dioxide or carbon dioxide-containing gas at a temperature of 50 -150 C. with a gas pressure of 20-165 pounds per square inch absolute, such that the mol ratio of carbon dioxide absorbed to total titratable alkali in the solution is in the range of 0.6 to 1.5. Since carbonation is carried out at elevated temperature and sodium bicarbonate is consumed in the reaction with sodium hydrosulfide in the stripping operation, relatively concentrated solutions of sodium salts (e. g., of the order of gm. per liter as NazO) may be treated without the precipitation of bicarbonate during processing. By total titratable alkali is meant the basicity equivalent to a standard acid titration to the methyl orange end point. In the case of soda smelts, this would include all of the sulfide, carbonate, and caustic soda, and one-half of the sulfite, and would exclude such salts as thiosulfate, sulfate, and chloride.

Under the stripping conditions of the invention, i. e., releasing the hot pressure carbonated solution into a vacuum, thus contacting it with steam at low temperature, very little bicarbonate decomposition occurs, with the result that the gas evolved is substantially hydrogen sulfide and water vapor. Steam is used as the stripping agent since a recovery of concentrated hydrogen sulfide gas can be effected simply by condensing the stream from the efliuent mixture of steam and hydrogen sulfide. Use of low pressure in stripping as applied to this operation is an important technical feature of our invention. Reduced pressure markedly improves the conversion and apparently reduces carbon dioxide losses from bicarbonate decomposition.

Irrespective of whether pure CO2 or flue gas is used for the carbonation, no attempt is made to completely elirninate sulfide in stripping since, as the stripping operation approaches completion, the efficiency of stripping with regard to steam consumption decreases tremendously.

The carbonation and stripping operations may be carried out in any tppe of equipment conventionally em ployed for gas absorption or stripping operations. Thus, for the carbonation and stripping, we may use packed columns, plate columns, spray columns, and continuous liquid phase columns. It is advantageous to use vessels or chambers for both the carbonation and stripping that permit easy separation of suspended or precipitated matter and which do not become plugged or fouled with separated lignin. Provision may be made for drawing off a lower layer or fraction containing the liquid or tar-like lignin product. Where the lignin is maintained in fine dispersion during carbonation and stripping, the lignin is preferably subsequently removed in solid granular form in separate equipment.

Flue gas may be used to advantage, requiring only slightly higher operating pressures or larger equipment than when using purer gas. Where a high degree of purity of the precipitated lignin product is desired, it may be advisable to purify the flue gas by removing suspended matter and scrubbing out any sulfur dioxide or other undesirable contaminant. Lime kiln gas, if available, may often be freed from suspended matter and used to advantage, since it will normally contain from 30% to 45% carbon dioxide.

flowjs' advantageouslyused. Undenthese,conditions, gas leayingthe top of the carbonator -is in contact with highly alkaline solutions (e. g. waste black liquor), Be cause of the-high alkalinity atthe point of gas exit, the amonnt of hydrogen sulfide leavingthe top of the carbonatorin dilute form is substantially nil. As -a result, the preponderant portion of the total hydrogen sulfide producedis liberated in the stripping operations in highlyconcentrated, readily usable form.

The solution obtained from the carbonation treatment is;introdueedinto a stripper which is maintained at a pressure substantiallybelow-the carbonation pressure and; below the'vapor pressure of water at the temperature of the carbonation, whereby a-fiashing" or sudden release of water vapor occurs accompanied by the simultaneous evolutionof a portion ofthe hydrogen sulfide with only'a relatively small, proportion of carbon dioxide. Although not; an. essential feature-of our process, it has been found desirable to allowthe flashing to occur in the vapor space at the top of a column, whereupon the remaining liquid passes down throughthis column in contact with-ascending steam supplied to the base of the column. This eflfectsithe efficient removal of the hydrogen sulfide formed in; the process. The hydrogen sulfide may be returned to the processto form digestion liquor.

In' a kraft pulping operation this hydrogen sulfide is convenientlyabsorbed in a solution of smelt either before or aftercausticization in order to raise the sulfidity to the: desired point, and the causticized, sulfide-enriched liquor reused as wood pulping liquor. Where a carbona tionpulping processinvolves an acid sulfite treatment of theiwood prior to the alkaline digestion, at leasta per! non. of the recovered concentrated hydrogen sulfide may. be readily burned to sulfur dioxidefor use 'in the preparationvofa the acid sulfite digestion liquor.

When the hydrogen sulfide has been effectively removed, the temperature of the stripped liquor is increased above 80 C.- to agglomerate the lignin, and then is cooled below thesintering'point to, say, around 40 C., in order to eliminate tackiness, whereupon the granular lignin maybe. readily filtered and washed. The lignin canbe used for-various'purposes 'or burned. The lignin may be dissolved in a portion of the original black liquor in. water-insoluble. form, is a partial sodium derivative.

The content ofjresidual sodium: in the precipitate is, however, small so thatthe. amount of sodium withdrawn from the pulping systemwith thev lignin is relatively. small.-

Where desired, the lignin precipitate recovered-by the carbonationprocessof the invention may be treated before dryingwith a sma 1l.portion of sulfuric or other mineral acid diluted withovater. Such treatment con-- verts the lignin to a sodium-free product which, after washing; and drying, isthermoplastic andmaybe usedfor. the manufacture of laminated paper plastics and other have the advantage of exceptionallylow melting points,-

e. g., around 158= C.

Alternatively, instead of removing the sodium remaining in the lignin precipitate from the carbonationtreao. ment toproduce insoluble thermoplastic products, additionalcausticsoda'may be added to the lignin precipitate toproduce water-soluble salts suitable for use as 'dispers.

ing.-.agents, for compounding with rubber lattices, etc. Thismaybe doneby adjusting the pH to about 9.5 or higherand suitablydrying the resulting solution to give a; dry, water-soluble powder.

Where itisnot desired to recover any or all of the.

lignin precipitate from the carbonation 'process' foruse as salable lignin products, this precipitate maybe sent to the furnace to be burned as a fuel, either as'such or after dissolving in a limited, amount of uncarbonated black liquor. We may recover'alignin precipitate representing 30% of the organic'material in the liquor which maybe burned as-a fuel or-recovered at total solidsanddried as a commercial product.

The data recorded inTable-Hare indicative of theresults which can be obtained by carbonating and stripping kraft black liquor. A few drops of a commercial antifoamant'were addedto reduce foaming and 'the carbon dioxide was introduced after the liquor had reachedthef desired temperature.

TABLE II carbonation of black liquors TestNo; 1 2 I a 4 5 a:

Liquor Data:

Cook N o Amount Treated, gms. Total Solids, Percent.

Total S, gms Carbonation: Temperature, C Residual Liquori Weight, g

a, gms Absorbent for Volatile Sulfur e,-m1 321 296 425 444 Total S, gms. (as analyzed)..-

(N aOH) Volum' Total 8, gms. (by ditferencey un, 0.355 0. 380 0i 292 0.177 0. 285 0.305 Apparent Sulfur Elimination, Percent 62. 6 59. 1 35. 8-

which has lay-passed the carbonation step, to obtain a solution ofaround 35% solids for burning, if desired.

The lignin may be recovered as a granular product very lowin sodium and low in sulfur; Whenhardwood is digested, the lignin has a relatively low melting point and because of these. properties has special industrial utilisation,

O sulfide sulfur content of the black liquor canbe reduced 7 The above data show that in the treatment of kraft liquors. which contain caustic soda and-sodium sulfide, the

lignin remains dispersed; above 80 C., the ligninsintersf "Theli'gnin recovered by the carbonationproeess, though or melt'si By the previously outlinedsequence' of operations wherein precipitation below the sintering point is followed by heating to agglomerate the lignin and then cooling, the lignin can be recovered as a granular material. Alternatively, by precipitating above the melting point, the lignin may be recovered as a liquid which cools to a firm plastic mass. An appreciable amount of odorus material was also removed during carbonation as evidenced by the pungent smell of the absorbent and the reduced odor of the black liquor.

Sulfur balances are summarized in Table II with the sulfur reduction computed by difference between the sulfur in the starting liquor and that in the residual liquor plus precipitated lignin. Approximately 10% of the sulfur was unaccounted for, presumably due to loss of organic sulfur compounds not completely absorbed in the caustic soda or to losses of volatile HzS during analysis.

Fig. 1 of the accompanying drawings is a graph showing the results of the potentiometric titration of raw black liquor, carbonated black liquor, and absorbent containing volatilized sulfide. (Determinations made on samples of test No. 6 of Tables II and III.)

'That all the sulfide sulfur was eliminated from the liquor by carbonation in test No. 6 and quantitatively absorbed in a suitable absorbent (NaOH) is evident from Fig. 1 showing the results of a potentiometric titration. The titration was carried out using a silver sulfide electrode and a glass reference electrode according to the method of Lykken, L, and Tuemmler, F. D. (Industrial and Engineering Chemistry, Analytical Edition 14, p. 67- 9 (January 1942)), as extended to black liquor by Barlew, P. B., and Pascoe, T. A. (Paper Trade Journal 122, No. 10, TAPPI Section, pp. 99102 (March 7, 1946)).

TABLE III Sulfur balance Sulfur Sulfur in- Sulfur Percent ill Evolved Sulfur Test N 0. Black By Reduc- Liquor Filtrate+Preoipitate Difierence) tion (c (e s) (c Sodium balances are shown in Table IV and indicate that very little sodium is associated with the precitated lignin. V The discrepancy between values for 1 and 2 appears to be due to a high figure for the original liquor.

TABLE IV Sodium balance Sodium Sodium inin Test No. Black Liquor Filtrate Precipitate Total ducing the. odor nuisance, and improving the flow properties of the resulting smelt when these liquors are furnaced for chemical recovery. Thus, the invention may be applied as a method of lowering the sulfidity of sultide-containing efliuents from the alkaline refining of pulp at elevatedtemperatures with solutions containing sodium carbonate and sodium sulfide. The treatment is particularly advantageous when applied to the efiiuent from the refining of sulfite pulp with sodium carbonate and sodium sulfide according to the process of copending application, Serial No. 290,209, of Arthur N. Parrett, filed May 27, 1952. The black liquor carbonation and stripping process is also applicable to treatment of sulfide-containing eflluents from the refining of substantially delignified pulp at elevated temperatures with solutions containing sodium sulfide and sodium hydroxide.

Where the effluents come from refining with mixtures of sodium carbonate and sodium sulfide; mixtures of sodium hydroxide and sodium sulfide, or mixtures of all three, the liquor, following removal of volatile sulfide, may be furnaced and burned to recover the inorganic components as a smelt. Where refining is carried out with a mixture of sodium sulfide and sodium carbonate, the concentrated hydrogen sulfide evolved in the carbonation treatment may be absorbed in a solution of smelt and reused directly in refining; Where refining is by a solution containing sodium hydroxide in addition to sodium sulfide, all or a portion of the smelt solution may be subjected to a causticizing treatment either before or after absorption of hydrogen sulfide recovered from the black liquor carbonation treatment.

Fig. 2 of the accompanying drawings illustrates by flowsheet an integrated cyclic operation embodying the invention. As illustrated in the flow-sheet, the black liquor carbonation process of the invention may be applied to provide a complete cyclic process of soda-base acid sulfite pulping and alkaline refining of sulfite pulp with re covery of both sulfite pulping and refining chemicals for use in the process. In this integrated process the chemicals used in alkaline refining are a mixture of carbonate and sulfide, as described in copending application of Arthur N. Parrett, Serial No. 290,209, filed May 27, 1952. The alkaline refining effiuent containting sodium sulfide is treated according to the invention by a pressure carbonation and vacuum stripping, effecting removal of concentrated hydrogen sulfide and producing a treated eflluent of lowered sulfidity. This treated efliuent is combined with the effiuent sulfite waste liquor from the acid sulfite pulping operation. The combined liquor is then evaporated and burned in a recovery furnace of the type used in conventional kraft operations whereupon a smelt is obtained consisting predominately of sodium carbonate and sodium sulfide. The concentrated hydrogen sulfide, recovered from the vacuum stripping, is of such strength as to readily support combustion, and is burned to provide sulfur dioxide for use in reconstituting soda-base acid sulfite wood digestion liquor (a mixture of sodium bisulfite and free sulfurous acid).

The smelt consisting predominantly of sodium carbon- I ate and sodium sulfide is dissolved and clarified and then is divided into two portions. One portion may be used directly in alkaline refining. The other portion may be treated by the ion exchange process of Letters Patent 2,656,244, of Kenneth Russell Gray and Hartzell Lance Crosby, to produce soda-base acid sulfite digestion liquor directly. According to this process, the solution containing this portion of the smelt is contacted with a carboxylic acid-type or phenolic-type cation exchange resin in hydrogen form whereby sodium is adsorbed on the resin and a mixture of CO2 and H28 may be evolved. The adsorbed sodium is subsequently desorbed from the resin by a sulfurous acid solution to produce soda-base acid sulfite digestion liquor. The aqueous effluent from this resin treatment containing unadsorbed cations and anions may be added to the smelt dissolving tank as shown in Fig. 1. Any mixture of CO2 and H25 evolved may be may be mixed with the flue "gas or other Co -containing;

gas going. to. the pressure carb onation operation.

Thisflintegrated process has a number ofimportant ad,- tfantages.. One of these is that by reduction of the sulfur. contentit permits practical furnacing: of sulfite waste; liquontoprovide afiuid smelt. In thisprocess, the sulfide contentof the-alkalineefiluent is first substantiallyeliniinated "by asimples olution carbonation using raw flue gas,, whereupon sulfite waste liquor may be added and-f the combinedfwaste efiiuents, still having a favorable so'dato sulfur ratio, may be practically evaporated and furnaced: One particular advantage of the integrated process isthat-the combined acid andalkaline effluents will be easier'tofurnace: than either effluent alone due to theliigli'inorganic content contributed by the alkaline efiluent and the high fuel value contributed by the sulfite waste liquor. Carbonatiomof the: alkaline eflluentpermits elimination of suflicient sulfur from. the. alkaline effluent, prior to combining with'th e' sulfurcontaining sulfite waste liquor that on combination the sodium-to-sulfur ratiods still favorable. for obtaining. afluid smelt without the nec'e'ssity of adding fluxing chemicals, such as sodium carbonate=prior tofurnacing. Inthis'wa'ynot only is a suitabledowi meltingsmeltobtained, but sulfur lossesin furnaeing are minimized. aswell.

Alternatively, in place of beingtreated" by said ion exchange 'process the portion. of smelt solution to be used forthe preparation of acid-sulfite digestion liquor may be treated" by the multistage solution "process of our abandonjed application, Serial No. 166,681, filed June 7-, 1950,

which subject matter. is, now. incorporated in application Serial No.- 415,857,.filed March, 12, 1954, now Patent No. 2,724,292,, granted, Nov. 22,.1955, to. produce a solution of'sulfide-freeisodium carbonate. This is accomplished by subjecting a portion. of the, dissolved smelt to a plurality of carbonation" treatments un'derp'ressure and at an elevated te'mp'erature with-agas consi'sting-at least" in part of carbon dioxide,; eachrsuch carbonation-treatment being followed by stripping under vacuum to remove volatile hydrogen sulfide in concentrated form" whereby sodium salts of carbonic acid are obtained as a solution substan tially free-from sulfide; This, byreaction with sulfur dioxide or sulfur dioxidercontaining gas, may be con verted to -soda-base acid-"sulfite digestion-liquor! Only a sufficient amount of flue gas need be-purified frorn'suspended solids to treat the portion of the soda chemicals which are to be converted into acid-sulfite digestion liquor. Frequently this amount of CO2 will be readily available in comparatively pure form from an alternative source, such as from the reaction of sodium carbonate with S02.

We claim:

1. In the process of preparing wood pulp wherein an alkaline digestion is carried out with a sodium sulfidecontaining solution and in which sulfide-containing waste black liquor is evaporated and burned to produce a smelt for the recovery of chemicals, the improvement which comprises carbonating the Waste black liquor with carbon dioxide under pressure and then stripping hydrogen sulfide from the carbonated liquor under a vacuum reducing the sulfide sulfur content of the liquor substantially, thereby reducing sulfur losses, recovering concentrated hydrogen sulfide, and minimizing obnoxious odors, and then evaporating the stripped liquor and burning the resulting concentrate.

2. In the process of claim 1, said pulping operation having been carried out on wood which has been subjected to a prehydrolysis treatment.

3. In the process of claim l,'carrying out the carbonation at a pressure of from 20 to 165 pounds per square inch absolute and at a temperature of from 50 to 150 C. and the stripping under a vacuum of from 5 to 29 inches of mercury.

4. In the process of claim 1, carrying out the pulping operation Wtih digestion" liquor in which the percentage ofthe totalNmOYpresentas Na2S isfrom 45 to 70%.

5. The:improvement'in waste liquor. treatment which.

comprises carbonating the sulfide containing; waste black liquor from analkaline digestion with carbonydioxide under pressure offrom 20 to 165 pounds pressure-per square inch absolute at a temperature below 70 C. to, prevent agglomeration of the lignin precipitated-indie. liquor, subjecting the liquor to astripping operation under a vacuum to remove concentrated hydrogen sulfide while maintaining the precipitated lignin infine: suspension, and: then evaporating the stripped liquor and-burning. the resulting. concentrate.

6. In the process of claim 5, wherein the black liquor was from an alkaline digestion carried out on wood which had been subjectedto aprehydrolysis treatment.

7. In the process of 'claim 5, while maintaining'agi tation, heating the stripped liquor to a-temperature above; the sintering. temperature or the moist dispersed ligninto efiect a. partial agglomeration, coolingthe liquor-below; thelignin sintering temperature toelirninate tackiness and provide a granular precipitate, filtering and washing said precipitate.

8. In the process oflclaim 5, wherein the kraft typepulping"operation:is-carriedoutwith wood which has been subjected to a pretty rolysis treatment, heating the stripped liquor whilemaintaining agitation to atemperature. above the sinteringtemperature of the moist dispersed lignin to eifecta partial agglomeration, cooling the liquor below the lignin sintering temperature to eliminatetackiness and provide a granular precipitate, filtering and washingsaid precipitate.

9. In the process of claim 5, wherein the stripped liquor is heated to a temperature of about 80 C. to agglomerate theprecipita-ted lignin;

10. In the process of --claim-9, pulping; ahardwood and; treating the separated lignin with a strong mineralacid followed by washing and-drying to produce a lignin'having a relatively low melting point.

1-1. In: akraft, typepulping operation in which the waste black liquor is'evaporated and? burned to produce a smelt for the recovery of chemicals, the'improvement' which comprises carbonating the'liquor-with carbon dioxide under-pressure while maintaining-precipitated lignin in suspensionin the liquor and Without; releasing any. appreciable amount of hydrogen sulfide, subjecting the carbonated liquor to a steam stripping operation for removal of concentrated hydrogen sulfide, concentrating and removing the lignin from the liquor, and then evaporating the stripped liquor and burning the resulting concentrate. I

12. The improvement in Waste liquor treatment which comprises carbonating the sulfide containing waste black liquor from an alkaline digestion with carbon dioxide under pressure of from 20 to 165 pounds pressure per square inch absolute at a temperature above C. to melt and liquefy a portion .of the lignin in the liquor, separating this lignin, subjecting the partially delignified liquor to a stripping operation under a vacuum to remove concentrated hydrogen sulfide, and evaporating the stripped liquor and burning the resulting concentrate.

13. In the processes of claim 12, dissolving the separated lignin in Waste black liquor to produce a waste liquor concentrate for burning.

14. In the process of preparing high alpha pulp and recovering chemicals, in which largely delignified pulp from the acid sulfite digestion of wood is subjected to an alkaline digestion with a solution containing sodium sulfide, the improvement comprising (1) treating the sulfide containing effluent from the alkaline digestion with a carbon dioxide-containing gas and subjecting the treated effluent to steam stripping under vacuum whereby c0ncentrated hydrogen sulfide is evolved and the sulfur content of the liquor is substantially reduced, and (2) evap- 1-1 orating and combusting the resulting solution with lowered sulfur content to produce a low melting smelt.

'15. In the process of preparing high alpha pulp and recovering chemicals, in which largely delignified pulp from the acid sulfite digestion of wood after chlorination and washing is subjected to an alkaline digestion with a liquor containing a mixture of sodium compounds consisting of sodium sulfide and at least one sodium compound selected from the group consisting of sodium carbonate and sodium hydroxide, said sodium hydroxide representing not more than 15% of the total sodium in the mixture, the improvement comprising (1) treating the sulfide-containing effluent from the alkaline digestion with a carbon dioxide-containing gas and subjecting the treated efiluent to steam stripping under vacuum whereby concentrated hydrogen sulfide is evolved and the sulfur content of the liquor is substantially reduced, and (2) evaporating and combusting the resulting solution with lowcred sulfur content to produce a low melting smelt.

16. In the process of claim 1, contacting the smelt solution with at least a portion of the elevated hydrogen sulfide to increase the sulfide content of the solution prior to use in the alkaline digestion.

17. In the process of recovering high alpha pulp and recovering chemicals, in which wood is digested in sodabase acid sulfite liquor to produce largely delignified sulfite pulp and in which said sulfite pulp after chlorination and washing is subjected to an alkaline digestion with a liquor containing a mixture of sodium compounds consisting of sodium sulfide and at least one sodium compound selected from the group consisting of sodium carbonate and sodium hydroxide, said sodium hydroxide representing not more than 15% of the total sodium in the mixture, the improvement comprising (1) subjecting the efiiuent from the soda-base acid sulfite digestion to steam stripping to remove a substantial portion of the free and loosely combined sulfur dioxide, (2) treating the sulfide containing efiluent from the alkaline digestion with a carbon dioxide-containing gas and subjecting the treated efiluent to steam stripping under vacuum whereby concentrated hydrogen sulfide is evolved and the sulfur content of the efiiuent is substantially reduced, (3) combining the treated alkaline effluent of lowered sulfur content with the stripped soda-base sulfite waste liquor in such proportion that the ratio of organic-to-inorganic solids is,

substantially increased over that in said alkaline effluent and' that sulfur-to-sodium ratio on an atomic basis does not exceed 0.25, and (4) evaporating and combusting the combined efiluents to produce a low melting smelt.

18. In the process of claim. 17, contacting the smelt solution with at least a portion of the evolved hydrogen sulfide to increase the sulfide content of the solution prior to use in alkaline digestion.

19. In the process of claim 17, the recovery of the sodium base used in the acid sulfite digestion comprising contacting a portion of the dissolved smelt wtih a carboxylic resin in hydrogen form whereby hydrogen sulfide and carbon dioxide are evolved and sodium is adsorbed on the resin, treating the resin containing adsorbed sodium with a sulfur dioxide solution whereby sodium is removed 15 from the resin and a solution of sodium bisulfite and sulfurous acid is produced for use in the acid sulfite digestion of wood.

References Cited in the file of this patent UNITED STATES PATENTS 1,566,118 Rawling Dec. 15, 1925 1,640,853 Richter Aug. 30, 1927 1,786,890 Braun Dec. 30, 1930 1,822,125 Blodgett et al. Sept. 8, 1931 1,864,619 Richter June 28, 1932 1,870,650 Richter Aug. 9, 1932 1,973,557 Bradley et al. Sept. 11, 1932 2,167,556 Smull July 25, 1939 2,228,976 Reboulet Jan. 14, 1941 2,249,174 Richter July 15, 1941 2,406,867 Tomlinson Sept. 3, 1946 2,470,764 Dunbar May 24, 1949 2,574,193 Savell Nov. 6, 1951 2,611,682 Mannbro Sept. 23, 1952 2,623,040 Keilen Dec. 23, 1952 FOREIGN PATENTS 137,831 Great Britain May 12, 1921 40 OTHER REFERENCES Rue et al.: Paper Trade J., Oct. 8, 1925, pages 52 and 53.

Rue et al.: Chem. and Met. Eng, October 1927, page 611. Schelhorn: Paper Trade J., Dec. 7, 1944, pages 39-44.

' (Copies in Sci. Lib.)

Mac "*7; 

1. IN THE PROCESS OF PREPARING WOOD PULP WHEREIN AN ALKALINE DIGESTION IS CARRIED OUT WITH A SODIUM SULFIDECONTAINING SOLUTION AND IN WHICH SULFIDE-CONTAINING WASTE BLACK LIQUOR IS EVAPORATED AND BURNED TO PRODUCE A SMELT FOR THE RECOVERY OF CHEMICALS, THE IMPROVEMENT WHICH COMPRISES CARBONATING THE WASTE BLACK LIQUOR WITH CARBON DIOXIDE UNDER PRESSURE AND THEN STRIPPING HYDROGEN SULFIDE FROM THE CARBONATED LIQUOR UNDER A VACUUM REDUCING THE SULFIDE SULFUR CONTENT OF THE LIQUOR SUBSTANTIALLY, THEREBY REDUCING SULFUR LOSSES, RECOVERING CONCENTRATED HYDROGEN SULFIDE, AND MINIMIZING OBNOXIOUS ODORS, AND THEN EVAPORATING THE STRIPPED LIQUOR AND BURNING THE RESULTING CONCENTRATE. 